A semiconductor memory device may be sorted mainly into a volatile semiconductor memory device and a non-volatile semiconductor memory device.
The volatile semiconductor memory device processes read and write rapidly, however, has disadvantage in that the stored data expires when an external power supply is terminated. On the other hand, a non-volatile semiconductor memory device is capable of retaining data even when the external power supply is interrupted. Therefore, the non-volatile semiconductor memory device is used to store data that must be saved whether or not power supply is interrupted. Examples of non-volatile semiconductor memory devices include a mask read-only memory (MROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), and an electrically erasable programmable read-only memory (EEPROM).
In general, a MROM, PROM and EPROM have systems that writing and deleting is relatively inconvenient for a general user to retrieve the stored data. However, as writing and deleting electrically is possible in an EEPROM, its application is extended to use in system programming which consistent renewal is required and in supplementary memory device. Particularly, a flash EEPROM may be applied to use as a large-capacity supplementary memory device as it is more highly integrated than a conventional EEPROM. Among flash EEPROMS, a NAND-type flash EEPROM (NAND-type flash memory, hereinafter) is especially highly integrated compared to other flash EEPROMs.
In a flash memory device, the data state which is capable of being stored in each memory cell is determined corresponding to the number of bit stored in each memory cell. A memory cell in which 1-bit data is stored in a single memory cell is called a single-bit cell or a single-level cell (SLC). And a memory cell in which a multi-bit data (e.g. more than 2 bits) are stored in a single memory cell is called a multi-bit cell, multi-level cell (MLC), or a multi-state cell. Recently, as demand for higher integration is increasing, multi-level flash memory capable of storing multi-bit data in a single memory cell is in active research.